The present invention relates to circuits for controlling the deflection of an electron beam in a cathode ray tube, and more particularly it relates to a circuit for deflecting an electron beam in a cathode ray tube to a predetermined angle in response to a predetermined input voltage, independent of the deflection sensitivity of the deflection means.
In cathode ray tubes, deflection means such as a magnetic field deflection coil are used to controllably deflect an electron beam. The angle of deflection of an electron beam traversing the magnetic field produced by the deflection coil, and having a particular acceleration potential, is a function of the value of the current flowing through the deflection coil. The relationship of the angle of deflection to the current flowing in the coil is defined by the transfer function of the deflection coil. The transfer function is generally a linear function for the intended range of operation of the particular deflection coil. The displacement of the electron beam observed at the screen of the cathode ray tube is therefore approximately proportional to the current flowing through the magnetic field deflection coil. The displacement caused at the screen by a unit change in the coil current is the deflection sensitivity of the deflection coil assembly.
Current is supplied to the deflection coil by a power amplifier circuit. By applying a generally sawtooth voltage waveform to the input of the power amplifier circuit, a repeating ramp current can be sent through the deflection coil, thereby causing the electron beam to trace and retrace a vertical or horizontal path on the screen of the cathode ray tube.
When a cathode ray tube having a deflection coil (a CRT/deflection coil assembly) is replaced, the deflection coil of the replacement assembly may have a different deflection sensitivity from that of the replaced coil. This is because deflection coils are manufactured within a specified tolerance range of the exact specifications. If no adjustment is made to the current supplied by the amplifier circuit to the deflection coil, the electron beam will scan a different path from the path produced by the replaced CRT/deflection coil assembly. Therefore, means must be provided for adjusting the current supplied to the coil to compensate for the variation in deflection sensitivity.
In the prior art, an adjustable resistance, or potentiometer, has been included in the circuit between the output of the amplifier and the input to the defelction coil. This potentiometer must be adjusted manually to divert the correct amount of current from the coil to compensate for differences in deflection sensitivity of different deflection coils.
One problem with the use of such a potentiometer is that undesirable noise is introduced. Another problem is the space taken up by the potentiometer. What is needed is a system for providing interchangeability of CRT/deflection coil assemblies without resorting to gain potentiometers on the assemblies.